Electrical signaling method

ABSTRACT

582,737. Secret transmission. PATELHOLD PATENTVERWERTUNGS- &amp; ELEKTRO-HOLDING AKT.-GES. April 5, 1944, No. 6377. Convention date, April 5, 1943. [Class 40 (iv)] In a system of secret telephony wherein the speech band is converted into a number of adjacent but transposed sub-bands of which the relative widths are constantly varying, the arrangement is such that at least one subband is inverted and at least one is not inverted. As shown in Fig. 4, the speech band e0 modulates successively a fixed frequency g1 and a varying frequency g2, the lower side-band being selected in each case to give the band e2. This then modulates a pair of frequencies g3, g4 spaced by the width of the band to give an inverted double-band e3 and also another pair of frequencies g5, g6 to give an erect doubleband e5. Complementary portions e4, e6 are selected from these double-bands and combined to form a double-band e7, from which the central portion is taken and shifted to produce a final band e10 comprising four subbands of varying width, two being erect and two inverted. In another arrangement of a similar character, different transpositions of the sub-bands are obtainable by a choice of carrier frequencies at the intermediate modulation stages. One or more of the sub-bands may be retarded ih relation to the remainder of the band ; for example, the selected portion e4 may be retarded by means of a telegraphone before combination with the portion e6. The speech band may be subjected to variable attenuation before scrambling. When the arrangement is applied to a multi-channel system, several speech channels may be grouped together for the purpose of scrambling.

N017. 19, 1946. Q GUNELLA 2,411,206

ELECTRICAL SIGNALING METHOD b3 TCI- 2- d 1 zwlal/ lganella BY J /ldf` YHTTORN E Y NOV. 19, 1946. Q GUANELLAy ELECTRICAL SIGNALING METHOD FiledMay 19, 1944 4 Sheets-Sheet 2 e] i g VE'Z I 3 f g 4 "//f/l/M "f///mIremn INVENTOR. nd/ella vf /zaf HTTORNE Y NV- 19 1945- G. GUANELLAELECTRICAL SIGNALING METHOD Filed .May 19, 1'944 4 Sheets-Shee. 4

M nK. m m6. Ww w TTORN EY Y two sub-bands of the scrambled signal.

Patented Nov. 19, 1946 il LZ FFICE ELECTRICAL SIGNALING METHOD GustavGuanella, Zurich, Switzerland, assignor to Radio Patents Corporation,New York, N. Y., a corporation of New York Application May 19, 1944,Serial No. 536,233 In Switzerland April 5, 1943 11 Claims.

or the like, is involved, and the main object of the invention is theprovision of improved and simplified arrangements for systems of thischaracter.

According to known methods of scrambling speech signals, the speechfrequency band is divided into several sub-bands of constant and equalwidth, these bands being interchanged according to a prescribed key. Inorder to increase the degree of secrecy, individual sub-bands may beinverted. A disadvantage of this method is the fact that, on account ofthe fixed frequency limits of the individual sub-bands, deciphering byrepeatedly reshifting the bands is greatly facilitated.

According to another known method, the speech frequency band is dividedinto unequal sub-bands of varying width which are transposed bymodulating them with auxiliary frequencies. The sub-band with theoriginal high speech frequencies then appears as the lower frequencyband, and the sub-band With the `original lower speech frequenciesappears as the upper frequency Iband of the scrambled signal, theoriginal lowest and the original highest frequencies adjoining eachother at the boundary between the A disadvantage of this method is thatdecoding is comparatively easy, if this frequency boundary is shiftedabout until the message is deciphered. Successive and repeatedapplication of this same method does not increase the degree of secrecy.

In order to avoid these disadvantages, the invention proposes a methodof scrambling speech signals and whereinv the signal which is to bescrambled is converted before transmission into a signal consisting ofseveral mutually interchanged sub-bands of the original frequency band,whose width is varied and at least one of which is inverted and at leastone is not inverted. This new method possesses the great advantage thatdue to the variable width of the individual bands, reshifting isrendered .diicult As a result of the continuously variable or veryfinely graduated adjustment of the band width, `a -much greater varietyof scrambling possibilities Ais obtained than is possible with bands ofconstant width.

The objects and novel aspects of the invention will `become more'apparent from thefollowing detailed description taken in reference tothe ,ac-

companying drawings forming part of this specication, and wherein:

Figure 1 is a diagrammatic representation explanatory of a known methodof speech scrambling by dividing a signal frequency band into sub-bandsof varying relative band widths.

Figures 2-5 are similar diagrams explanatory of the improved methodaccording to the invention.

Figure 6 is a block diagram of a system for carrying out a speechscrambling method of the type illustrated in Figure 5; and

Figure 7 is a further diagram illustrating the reproduction of a messagescrambled in accordance With Figure 5.

Referring to Figure l, an loriginal speech frequency band is shown atbo. By modulating band bo with the constant auxiliary frequencies f1, f2and suppressing the lower side bands or difference Vfrequencies of bothmodulation products, the frequency bands b1, b2 are obtained. A furthermodulation of bands b1 and b2 with the variable auxiliary frequency fsresults in an intermediate signal b3 after suppression of the sumfrequencies of the modulation product. `The frequencies of the signal bsindicated by the broken lines are suppressed by a Vband filter having afixed band pass characteristic. A further modulation of band b3 with theconstant auxiliary frequency f4 then produces as final dilferencefrequencies the scrambled signal b4. The original signal amplitudes areindicated in all signals by the height of the shaded areas, assuming alinear variation as a function of frequency of the signal bo.

In a system of vthis type, therefore, the signal frequency band isdivided into a plurality of subbands mutually interchanged in respect totheir initial relative position and having relatively varying frequencysuch that the total frequency band remains the same and equal to that ofthe original signal.

The signal b4 may be easily deciphered by reshifting it according to thesame method until the original signal bo is obtained. A furtherapplication of the same scrambling'method does not increase the secrecy,because no further subdivision of the frequency band is obtained, andthe signal ba obtained from b4 by a further modulation process withfrequencies ft-fa corresponding to fi-f4 mai7 readily be deciphered by asimple reshifting of the frequency bands.

The secrecy of such a system is greatly improved according'to thepresent invention by subjecting lat vleast one sub-band .of theintermediate signal `ba Vto .a 'frequency inversion. Thus, as

2,411,2o6 Y f shown in Figure 2, band b3 may for instance be dividedinto the sub-bands c4 and c5. Inversion of the latter then results inthe band cs. By adding the frequency bands c4 and ce, the band c'z isobtained and finally by reshifting the frequency band to the initialposition, a nal scrambled signal cev is produced. It is obvious that thescrambled signal cg may be subjected to further scrambling processes,Such as by repeating the above method, whereby contrary to a repeatedscrambling process of Figure 1, an increased degree of secrecy isobtained.

Alternatively, it is possible to obtain from signal b3, by inverting onesub-band thereof, a new signal d4 which is shifted to its initialposition d5. In this case, too, repeated application of the scramblingprocess will result in an increase of secrecy.

According to another modification of the invention, a sub-band of theintermediate signal b3, for instance band c5 of Figure 2, may beretarded for a definite interval of time. A signal is then obtainedwhich, in order to increase the secrecy, can be subjected a number oftimes to the aforementioned scrambling processes.

Another method of carrying out a scrambling process according to theinvention is illustrated by Figure 4. The intermediate frequency band e1is obtained from the original speech frequency band en by modulating itwith the constant frequency g1. A further modulation with the variablefrequency gz produces the new intermediate frequency band e2 which isshown for an average position of 92. A further modulation of e2 with theauxiliary frequencies g3 and g4 then results,

after suppression of the sum frequencies, in a new intermediatefrequency band e3 which is inverted with respect to e0. A limited bande4 is segregated from e3 by means of a filter of corresponding band passcharacteristic. By modulating e2 with the further constant auxiliaryfrequencies gs and ge a further new intermediate frequency band es isobtained which, after the suppression of a sub-frequency range by asuitable filter,

results in the signal es. By adding e4 and es the intermediate frequencyband e7 is produced. Renewed modulation with the optional auxiliaryfrequency g8 results in the difference frequency e8 from which thesub-band e9 is ltered out. Finally, by modulating with the auxiliaryfrequency gio, the scrambled signal ein in the original frequency bandis obtained. It will readily be seen 'that em contains all components ofthe original signal e as transposed frequencies.

As the frequency of g2 i-s varied, the sub-bands in ein move accordinglyin such a manner that the frequencies which pass beyond the upper limitof the frequency band reappear at the lower part of the band and, due tothe inversion of part of the sub-bands, it will- :be practicallyimpossible to reproduce the original signal by reshifting of the bandsby an unauthorized receiver.l

A further advantage of the scrambling method described hereinabove isdue to the fact that by employing the same a number of times insuccession, it is possible to increase the degree of secrecy to anydesirable degree.

An alternative manner of carrying out a scrambling method according tothe invention is illustratedin Figure 5. By modulating the speech.frequency band Zo with a frequency h1, the first intermediate signal Z1is obtained and as a result of a further modulation with a frequency h2,a second intermediate signal Z2 is produced. Further modulation with avariable auxiliary frequency h3 then results in the intermediate signalls. The low frequency sub-band of Z3 lying below the xed boundaryfrequency 1c segregated by means of a similar cut-off filter is movedupwards again by modulating it with h3 to produce the lower sub-band ofthe new intermediate signal Z4. IThe upper `sub-band of 14 resultsf'romthe inversion of the upper sub-band of la by modulating it with h4.. Anintermediate signal l may also be formed by modulating the lowersub-band of la with a frequency h5 and modulating the upper sub-band ofZ3 with the frequency h3. The intermediate signals Z4 and l5respectively thus eachY contain two signal frequency bands of variablewidth, one of these bands being inverted. If the same process isI"epeated with the auxiliary frequency he and hq or ha, this will resultin the intermediate frequency bands le and lv, respectively, and finalbands Z8, Z9, Zio, In. When the process is repeated a third time, theintermediate signal ly and the scrambled signal Zz are obtained, whichin the case under consideration occurs in eight different combinations.

With this method a deciphering can be rendered more difficult byrepeating the procedure,

without the clarity of the transmitted speechv being thereby affected toany noticeable extent.

When modulating with variable frequencies, the lower side bands havealways been used as an intermediate signal for scrambling purposes. Theupper side band-s may, however, also be used for the same purpose. Theshifting of the frequencies by variable amounts may also be accomplishedby a modulation frequency whose frequency lies above the range of thesignal frequency which is to be shifted.

Figure 6 illustrates in block diagram form an arrangement by means ofwhich the method according to Figure 5 may be realized. The modulatorsand band filters are indicated by letters M and F, respectively, and thecut-off filter-s by W, while the reference letters l for the frequencybands and h for the modulation frequencies are the same as those inFigure 5. The change-over 7 switch S enables the frequencies h3 and h4or ha and h5 to be employed for modulation as required,

whereby bands Z4 and l5, respectively, are formed.

Similar change-over switches (not shown) may be provided after thesecond and third shunting switches by-mean-s of which it is possible tochoose between the intermediate signals le, lio and 19,111,

respectively, and the desired constellation of the bands ly.

When applying the method according to the invention, it is desirablethat the auxiliary frequencies h3, hs and hg should be finely graduatedor made constantly variable so that the band limits in the scrambledsignal can be shifted as desired. A scrambled signal is thus obtained,the 50 sub-bands of which have an unequal band width. In order toreconstruct .the original signal, the conversions employed during thescrambling process are eliminated gradually by reversing the operations,Figure 7 illustrates one way of achieving this, the same frequencies(these being indicated again by vertical arrows) and the same y nals isan inverted signal. An advantage of these partial retardationsis due tothe fact that the temporal course of the energy in the scrambled signaldiffers from that of the original signal, so that unauthorizeddeciphering by studying the energy rhythm is impossible. The energyrhythm can also be made to differ` from its original course if thesignal to be scrambled is subjected to a variable damping which may ormay not depend upon frequency.

The method according to the invention may also be applied tomulti-channel transmission. In this case, it is not necessary for eachchannel to be scrambled, it being suiiicient if several speech channelsare grouped together for the purpose of l scrambling, so that themessage frequency band formed from several channels is subjected as aWhole to one of the` scrambling processes described.

YI claim:

g 1. In a secrecy signaling system, the method consisting in separatelymodulating a pair of fixed carrier frequencies by a signal frequencyband, said carrier frequencies being separated by a distance equal tothe width `of said signal band, modulating a further periodicallyvariable carrier frequency by the resultant side bands located at thesame .side of said fired carrier frequencies to produce a resultantintermediate band comprising two adjacent signal bands movingv back andforth along the frequency scale at the rhythm of said variable carrierfrequency, and segregating from said intermediate band a band equal tothe original signal band and lyingl within fixed points on theYfrequency scale located at equal distances from the middle of saidresultant band in the position corresponding to the mean value of saidvariable carrier frequency- 2. In a secrecy signaling system, the methodconsisting in separately modulating a pairof fixed carrier frequenciesby a signal frequency bad,said carrier frequencies being separated by a-distance equal to the width of said signal band, modulating a furtherperiodically variable carrier frequency by the resultant side bandslocated at the same side of said fixed carrier frequencies to produce aresultantv intermediate band comprising two adjacent signal bands movingbackrand forth .along the frequency scale at the rhythm of said.variable carrier frequency, segregating from s aid intermediate band aband equal to the original signal band and lying within fixed points onthe frequencyscale located at equal distances from into a plurality ofsub -bands rearranged with respect to their originall order to produce ascram- -bled signal of band widthequal to the band width of the originalsignal band, means for inverting at least one of said sub-bands, andfurther means for continuously and periodically shifting the individualfrequencies of the scrambled signal band between the upper and lowerlimits thereof and with the frequencies passing beyond the upper endreappearing at the lower end' of the band.

5. In a secrecy signaling system, a source of signals comprising a bandof component frequencies, means to provide a pair of Xed carrierfrequencies located outside of and separated by a distance equal to saidsignal band, means for modulating each of said carrier frequencies bysaid signal band, a source of periodically variable carrier frequency,means for modulating said variable carrier frequency by the resultantside bands 0btained from said rst modulating means and located at thesame side of said fixed frequencies to produce a resultant intermediatefrequency band comprising two adjacent signal bands moving back andforth along the frequency scale at the rhythm of said variable carrierfrequency, and a band pass filter having fixed lower and upper frequencylimits comprising a range equal tosaid signal frequency band and spacedby equal distances from the center of said intermediate frequency bandin lthe position correspondingto the mean value of said variable carrierfrequency.

6. In a secrecyv signaling system; a sourceof signals comprising a bandAof component frequencies, means to provide a pair of fixed carrierfrequencieslocatedoutside of yand separated by a distance equal to said`signal, band, means for modulating each of said carrier frequencies bysaid signal band, a source of periodically variable carrier frequency,"meansy for kmodulating said variable carrierv frequency by the resultantside bands obtainedfrom said firstl'modulating means and locatedat thesame side of said fixed frequencies to produce a resultant intermediatefrequency band comprising two adjacent signal bands moving back andforth along the frequency scale atfthe rhythm of said variable carrierfrequency, a band pass filterhaving fixedlower and the middle of said'resultant bandin the position corresponding, to the mean value of saidvariable carrier frequency, whereb-y the segregated band is divided intesub-bands'of constantlyvarying relative band width, andfrequency-inverting at least one of said sub-bands.

` 3. In a secrecy signaling system, a source of signals comprising aband of component frequencies, means for subdividing said signal bandinto a plurality of sub-bands and for rearranging the order of saidsub-bands to produce,V a scrambled signal of band width equal to theband width of the original signal, means for inverting at least one ofsaid sub-bands, and further means for continuously shifting theindividual frequencies of the scrambled signal band within the upper andlower frequency signal limits thereof k and with the frequencies passingbeyond the upper end reappearing at the lower end of the band.

Ll. In a secrecy signaling system, a source of signals comprising a bandof component frequencies, means for subdividing said signal band upperfrequency limits 'comprising a range equal to said signal frequency bandand spacedA byY equal distances from the center of said intermediatefrequency bandwin the position oorrespondingto. the mean value lof saidvariablecarrier frequency, p whereby, the band segregated by said lteris divided into sub-bands of continuously relatively varying band width,and means for inverting at.

least. one of said sub-bands. Y

7L In a secrecy signaling system, a source of signals comprising a bandof component. fre,- quencies, means to provide apair of fixed car-y rierfrequencies located outside of Vand separated by'a distance equal-tosaidsignalA band, means for modulating each of said carrier frequencies bysaid signal band, a source of periodically variable carrier frequency,means for modulating said variable carrier frequency by the resultantside bands obtained from said first modulating means and located at thesame side of said fixed frequencies to produce a resultant intermediatefrequency band comprising two adjacent signal bands moving back andforth along the frequency scale at the rhythm of said variable carrierfrequency, a band pass filter having xed lower and upper frequencylimits comprising a range equal to said signal frequency band and spacedby equal distances from the center of said intermediate frequency bandin the position corresponding to the mean value of said variable carrierfrequency, further band pass filter means having a band width less thansaid first band pass filter for dividing said intermediate frequencyband into at least two partial bands, and means for inverting at leastone of said partial bands and recombining it with the remainingnon-inverted partial band. l

8. In a secrecy signaling system, a source of signals comprising a bandof component frequencies, means to provide a pair of fixed carrierfrequencies located outside of and separated by a distance equal to saidsignal band, means Vfor modulating each of said carrier frequencies bysaid signal band, a source of periodically variable carrier frequency,means for modulating said variable carrier frequency by the resultantside bands obtained from said first modulating means and located at thesame side of vsaid fixed frequencies to produce a resultant intermediatefrequency band comprising two adjacent signal bands moving back andforth along the frequency scale at the rhythm of said variable carrierfrequency, a band pass filter having fixed lower and upper frequencylimits comprising a range equal to said signal frequency band and spacedby equal distances from the center of said intermediate frequency bandin the position corresponding to the mean value of said variable carrierfrequency, a further band pass filter having a -band width being afraction of the band width of and symmetrically located with respect tosaid iirst band pass filter to produce-a pair of sub-bands from saidintermediate frequency band, and means for frequency-inverting at leastone of said last-mentioned sub-bands and recombining it with the othersub-band.

' 9. In a secrecy signal system, a source of sigdistance equal to saidsignal band and v'located symmetrically to said first pair of fixedcarrier frequencies with respect to said side bandin the positioncorresponding to the mean value of said variable carrier frequency,means to modulate each of said pairs of fixed carrier frequencies by lsaid'side band and for segregating a pair of second modulation sidebands from the modulation product, each of said second side bandscomprising two adjacent signal side bands with the signal bands of oneside band being inverted relative to the signal bands of the other sideband, and further means for segregating from each of said pairs ofsecond side bands a predetermined subband and for combining resultantsub-bands to produce a final scrambled signal of band width equal to theoriginal signal band and having subbands varying continuously betweenthe upper and lower frequency limits thereof.

10. In a secrecy signaling system, a source of signals comprising a bandof component frequencies, a first source of periodically variablecarrier frequency, means for modulating said carrier frequency by saidsignal band, means to divide a resultant modulation side band at a fixedfre-- quency to produce a pair of sub-bands of continuously varyingrelative band width, means for inverting at least one of said sub-bandsand combining it with the other sub-band to produce an intermediatescrambled signal, a second source of periodically variable carrierfrequency differing from said iirst source by a predetermined frequencydifference, means for modulating said second carrier frequency by saidintermediate signal, and furthermeans for dividing a resultantmodulation side band at said fixed frequency to produce a secondintermediate signal of width equal to the original signal frequency bandand comprising three sub-bands of continuously varying relative bandwidth.

11. In a secrecy signaling system, a source of signals comprising a bandof component. frequencies, a first source of vperiodically Variablecarrier frequency, means for modulating said carrier frequency by saidsignal band, means to divide a resultant modulation side band at a fixedfrequency to produce a pair of sub-bands of continuously varyingrelative band width, means for inverting at least one of said sub-bandsand combining it with the other sub-band to produce an intermediatescrambled signal, a second source of periodically variable carrierfrequency differing from said first source by a predetermined frequencyVdifference, means for modulating said second carrier frequency by saidintermediate signal, further means for dividing a resultant modulationside band at said fixed frequency to produce a second intermediatesignal of width equal to the original signal frequency band and,comprising threesub-bands of continuously varying relative band width,and means for further successively modulating the resultantkintermediate signals against variable carrier frequencies differingfrom theV preceding carrier frequencies and for'deriving resultant sidebands to produce a final scrambled signal of band width equal Atotheforiginal signal band and comprising Ya ldesired number of sub-bandssome of which are inverted and all of which vary continuously inrelative width within the upper and lower frequency limits of thescrambled signal.

` GUSTAV GUANELLA.

